The advantages of acrylic polymers as viscoelastic bases for pressure-sensitive adhesives are well known in the art. U.S. Pat. No. Re. 24,906 (Ulrich) cites many examples of these products. Initially, such compositions were made via solution polymerization. However, such methods of polymerization involved the use of large amounts of organic solvents. This was undesirable for both safety and economic reasons. Further, the necessity of an excess of such solvents in order to render the polymers transferable between containers drastically limited the handleability and transportability of the polymers.
Various methods of suspension or emulsion polymerization for copolymer pressure-sensitive adhesives have been disclosed in the art. Emulsion polymerization uses water as the reaction medium, and the polymerization takes place within a micelle which easily dissipates the exotherm due to the heat of polymerization. Because water is the solvent, the resulting emulsion is safer to handle. However, the pressure-sensitive adhesives made via emulsions differ in performance properties and/or coatability from those made via solutions depending on the techniques employed, i.e., emulsion-formed adhesives frequently exhibit decreased adhesion.
U.S. Pat. No. 3,691,140 discloses pressure-sensitive adhesive microspheres having average diameters of from 5 micrometers to about 150 micrometers formed via suspension polymerization in the presence of large amounts of surfactant. Upon standing after polymerization, the microspheres will separate into two or more phases. These microspheres are crosslinked to maintain their beaded configuration when coated. This results in a rough coating rather than a continuous coating.
U.S. Pat. No. 4,988,567 and U.S. application Ser. No. 660,358 now U.S. Pat. No. 5,053,436 discloses infusible and solvent insoluble hollow acrylate microsphers having multiple small voids and the processes for making them. The references teach processes including a process of polymerization of a water in oil in water emulsion, known as multiple emulsions, in which the concentration of emulsifier in water should be greater than its critical micelle concentration.
Suspension polymerization of pressure-sensitive adhesives without crosslinking and in the absence of large amounts of surfactant has been attempted, but keeping the polar monomers such as acrylic acid in the organic phase long enough for copolymerization with the alkyl acrylate monomers has been difficult. Japanese Laid-Open patent application No. 57-42778, published Mar. 10, 1982, discloses a pearl-shaped pressure-sensitive adhesive polymer obtained by suspension polymerization of, e.g., methacrylic acid in alkyl acrylates and a specially formulated dispersing agent consisting of a copolymer made of from 80-99.5% of a hydrophilic monomer and from 0.5-20% of a hydrophobic monomer. Conventional dispersing agents are disclosed which cause the resulting pearl-shaped polymers to block or agglomerate during polymerization. Furthermore, it is disclosed that even the specially formulated dispersing agents described may decrease the adhesive properties of the copolymer.
German Patent No. 24 55 133, published Mar. 4, 1982, discloses suspension polymerization of acrylate monomers into bead-shaped polymers. The polymer beads are coated with a crosslinking agent after polymerization to render them sensitive to radiation.
No water-soluble polar monomers are used in the adhesives. The beads themselves need not even be tacky; low adhesion values are disclosed as is the addition of conventional tackifying agents.
U.S. Pat. No. 3,786,116 discloses use of macromolecular monomers (macromers) in water-based polymerization systems. No pressure-sensitive adhesive systems are disclosed.
U.S. Pat. No. 4,554,324 discloses hot melt pressure-sensitive adhesives formed by copolymerization of macromers with alkyl acrylates to improve shear holding properties. Emulsion polymerization is disclosed, but not exemplified or preferred; all examples utilize solution polymerization.
U.S. Pat. No. 4,551,388 similarly discloses incorporation of from 1% to 30% methacrylate macromonomer by copolymerization.
U.S. Pat. No. 4,851,278, assigned to the assignee of the present application, discloses solvent polymerization of pressure-sensitive adhesives comprising zinc carboxylates and polystyrylethyl methacrylate macromonomers.
U.S. Pat. Nos. 4,952,650 and 4,833,179 assigned to the assignee of the present case, both incorporated by reference herein relate to a method for suspension polymerization of a pressure-sensitive acrylate copolymer bead having a glass transition temperature of 0.degree. C. or less. The method comprises making a monomer premix comprising an acrylic acid ester of non-tertiary alcohol, the alcohol having from 1 to 14 carbon atoms, with the average number of carbon atoms being about 4 to about 12, a polar monomer copolymerizable with the acrylic acid ester, a chain transfer agent, a free-radical initiator, and a modifier moiety selected from the group consisting of 2-polystyrylethyl methacrylate macromolecular monomers, reactive zinc salts and hydrophobic silicas. The premix is then combined with a water phase containing a sufficient amount of suspending agent to form a suspension. The suspension is concurrently agitated and polymerization of the polymer premix is permitted until polymer beads are formed. The polymer beads are then collected. The amount of the modifier moiety must be sufficient to render the copolymer bead non-agglomerating at room temperature and safely handleable and transportable. The filtration products comprising the beads and water are easily handleable and easily and safely transportable. They are surprisingly free flowing, and do not agglomerate into unmanageable masses. They may be easily poured into tanks for transportation to distant manufacturing facilities. The beads are storage stable, and may be placed into storage tanks for long periods of time without undergoing physical or chemical degradation. Because the beads are stored wet rather than in organic solvents, they may be transported and stored without the elaborate safety and environmental procedures necessary when organic solvents are present. When dried, the polymers are inherently and permanently tacky.
Monomers with considerable solubility in water such as methacrylic acid may be copolymerized with acrylic or methacrylic ester by suspension polymerization. This is possible because, during the polymerization, the water miscible monomer may diffuse into the organic phase for copolymerization. But, as taught in U.S. Pat. No. 4,833,179, a zinc compound, such as zinc oxide must be used as a co-suspension agent to prepare a stable suspension.
It is not possible to fully react monomers having high water solubility, such as acrylic acid, acrylamide, etc. with water-insoluble monomers, such as acrylic or methacrylic esters via aqueous suspension polymerization. However, U.S. Pat. No. 4,833,179 teaches that, in such polymerizations, added electrolytes produce a salting-out effect and complete copolymerizations are possible. The brine solutions necessary for such reactions, however, render these reactions less desirable from an environmental viewpoint.
Therefore, a need exists for a method of making suspension PSA beads that can be prepared in the absence of a modifier moiety such as zinc oxide.
A need also exists for a method of making suspension PSAs in which highly water-soluble monomers can be incorporated into the polymer backbone in the absence of a brine solution.
A need also exists for a method of making suspension PSA beads that have improved adhesive properties, e.g., both high shear holding strength and high peel adhesion values.
Adhesives having high shear values have a high level of cohesive strength due to their crosslinked nature and/or high molecular weight which contributes to their high level of internal strength.
Adhesion relates to the wetting properties of an adhesive. In the past, in order to obtain good wetting properties, a low molecular weight adhesive was needed since a low molecular weight adhesive has better flow and is thus more able to "wet" and thus adhere to a substrate. However, low molecular weight adhesives have poor internal strength and thus poor shear.
In the past, it has been possible to have a high molecular weight adhesive having good shear strength but low adhesion. Also, it has been possible to have a low molecular weight adhesive having good wettability and thus high adhesion but a low shear strength due to its low molecular weight.
A need therefore exists for an adhesive which combines the advantages of both low molecular weight and high molecular weight adhesives (i.e., good adhesion and good shear). We have found such a method of PSA bead preparation.
The present invention involves a combination of emulsification and suspension polymerization processes such that monomers having high solubilities in water can be incorporated into the polymer backbone of the PSA beads. The PSA beads of the present invention have a distinctive morphology which is different from the suspension PSA beads disclosed in U.S. Pat. Nos. 4,833,179 and 4,952,650. The morphology of the suspension beads of the present invention is termed "structured beads" to differentiate from conventional suspension beads. Another unexpected benefit of the suspension beads prepared according to the method of the present invention is the improvement in adhesive properties. Although, not wishing to be bound by theory, we postulate that, during polymerization in the water-in-oil emulsion in water suspension, the polar monomers in the emulsion particles will graft onto or form blocks with the acrylate PSA bead backbone. The blocks or grafts thus formed contribute the desired high shear properties and the acrylate PSA backbone contributes high adhesion values desired in a good PSA.
Another benefit of the suspension process of the present invention is that suspension PSA beads can be prepared with or without zinc oxide. Zinc-free PSA can be advantageous in some applications.
In practicing the present invention, it is essential to employ both emulsification and suspension polymerization processes to prepare structured beads which have hydrophilic polymer domains, of the size typically associated with emulsion particles, within the suspension beads.